As a method for manufacturing a metallized ceramic substrate, a co-firing method (simultaneous firing method) and a post-firing method (sequential firing method) are known. In the co-firing method, a metal paste layer is formed on an raw ceramic substrate precursor called a green sheet to prepare a metallized ceramic substrate precursor and fire it. In this method, the green sheet and the metal paste are fired simultaneously.
In the post-firing method, a metal paste layer is formed on a ceramic substrate obtained by firing a green sheet to prepare a metallized ceramic substrate precursor and fire it. In this method, the green sheet and the metal paste layer are fired sequentially. Both of the methods allow formation of a metallized pattern on the ceramic substrate, and the substrate thus obtained is mainly used as a substrate for mounting an electronic component.
For example in the case of forming a wiring by the co-firing method, the green sheet is likely to shrink unevenly upon firing. If a square green sheet is sintered for example, a central portion on each side thereof shrinks slightly to warp inward, resulting in deformation of the substrate into a star shape. Therefore, when many metallized patterns having the same shape are formed on one green sheet, the shape of the pattern inevitably changes slightly depending on the location of the patterns.
On the other hand, in the case of forming a metallized pattern by the post-firing method, an electroconductive paste is applied directly on a ceramic substrate and dried, and thereafter fired to form the metallized pattern. When printing (firing) the electroconductive paste layer, the electroconductive paste layer shrinks in its thickness direction, but hardly shrinks in its planar direction. Thus, there is not a problem that the shape of the pattern changes depending on the location thereof, as seen in the co-firing method.
In this viewpoint, the post-firing method can be said to be suitable for forming a fine pattern on the substrate. However, as the size of the electronic components to mount on the substrate decreases, further improvement of precision and fineness of the metallized pattern on the substrate is required; and conventional manufacturing methods by the post-firing method cannot meet such requirements. For example, even when a metal paste is applied in an intended shape of a circuit pattern in the post-firing method, the metal paste sometimes flows or spreads before sintered, causing difficulty in achieving a fine pattern.
In the post-firing method, the metal paste is usually screen-printed on the sintered substrate and fired, to thereby form a metal pattern; and a titanium powder and the like for example is included in the metal paste in order to improve adhesion between the metal pattern and the substrate (see Patent Document 1). Since the method described in Patent Document 1 originally intends to form a metal pattern with only a metal paste layer, the metal pattern layer is thick and the cross section of the metal pattern formed is in a trapezoidal shape with a long base as shown in FIG. 1, causing further difficulty in forming a fine pattern. In the method, firing is performed after a metal paste layer not containing a titanium powder is formed on a metal paste layer containing a titanium powder. Upon firing, titanium will be dispersed in the area not having the titanium powder as well and will end up existing in the whole metallized pattern, though in varying concentration, thereby tends to cause degradation of the electrical conductivity. In this point, the method still needs improvement.
There is a method of forming a metallized pattern by an etching process (Patent Document 2), as a technique that allows further improvement of the pattern density than the method of forming the metallized pattern by the screen printing described above. In this method, a metallized substrate is produced by: forming sputtered titanium and copper layers on a sintered substrate; forming a copper plating in a through-hole; forming, by a photolithographic method, a resist pattern on a part where a metallized pattern is not to be formed; plating copper, nickel, and gold in the mentioned order; removing the resist; and removing the sputtered titanium and copper layers by etching.